Unexamined Japanese Patent Publication No. Hei. 9-237938 has disclosed that a (111) surface of metal nitride having a rock salt structure as an undercoat layer is used as a substrate in order to obtain group III nitride compound semiconductor layers with good crystallinity. That is, in this publication 9-237938, metal nitride having a rock salt structure is used as a substrate so that group III nitride compound semiconductor layers are grown on the (111) surface of the substrate.
A substrate for semiconductor device needs to have characteristic (stiffness, impact resistance, and so on) for retaining the function of the device. When the substrate is made of metal nitride, it is conceived that the substrate needs to have a thickness of not smaller than 50 μm to retain the characteristic.
Metal nitride with such a large thickness has been never provided as a raw material for semiconductor-producing industrial products.
Therefore, an object of the present invention is to form group III nitride compound semiconductor layers with a good crystal structure by use of raw materials industrially easily available. Accordingly, the semiconductor device produced by the present invention has semiconductor layers with a good crystal structure and can be produced inexpensively.
According to a different aspect, another object of the present invention is to provide a group III nitride compound semiconductor device having a novel structure, and a method of producing the device.